The invention relates to a process for converting cycle oils produced in catalytic cracking reactions into olefinic naphthas. More particularly, the invention relates to an out-board catalytic cracking process for converting a catalytically cracked light cycle oil (also referred to herein as light cat cycle oil or LCCO) into light olefins (C2-C5) and catalysts used in such processes.
Cycle oils such as LCCO produced in fluidized catalytic cracking (FCC) reactions contain two-ring aromatic species such as naphthalene. The need for blendstocks for forming low emissions fuels has created an increased demand for FCC products that contain a diminished concentration of multi-ring aromatics. There is also an increased demand for FCC products containing light olefins that may be separated for use in alkylation, oligomerization, polymerization, and MTBE and ETBE synthesis processes. There is a particular need for low emissions, high-octane FCC products having an increased concentration of C2-C4 olefins and reduced concentration of multi-ring aromatics and olefins of higher molecular weight.
Hydroprocessing a cycle oil and re-cracking hydroprocessed cycle oil results in conversion of the cycle oil to a motor gasoline blend-stock. In some conventional processes, the hydroprocessed cycle oil is recycled to the FCC unit from which it was derived. In other conventional processes the hydroprocessed cycle oil is re-cracked in an additional catalytic cracking unit, sometimes referred to as an outboard catalytic cracker.
Some conventional hydroprocessing operations hydroprocess cycle oil such as LCCO to only partially saturate bicyclic hydrocarbons such as naphthalene to produce tetrahydronaphthalene. Hydroprocessing and subsequent LCCO re-cracking may occur in the primary reactor vessel. Hydroprocessed LCCO may also be injected into the FCC feed riser at a point downstream of feed injection to provide for feed quenching.
Unfortunately, such re-cracking of hydroprocessed LCCO results in undesirable hydrogen transfer reactions that convert unsaturated species such as tetrahydronaphthalene into aromatics such as naphthalene and decreases the olefin yield from the outboard catalytic cracker.
There remains a need, therefore, for new processes for light olefins produced from hydroprocessed cycle oils such as LCCO.
Accordingly, one embodiment of the present invention comprises a fluid catalytic cracking process that comprises the steps of: (a) contacting a FCC feed with a catalytic cracking catalyst in a first catalytic cracking stage under catalytic cracking conditions to produce cracked products; (b) separating at least a cycle oil fraction containing aromatics from the cracked products; (c) hydrogenating at least a fraction of the aromatics in the cycle oil fraction in the presence of a hydrogenating catalyst under hydrogenation conditions to form a hydrogenated cycle oil; and, (d) contacting the hydrogenated cycle oil with a catalytic cracking catalyst under catalytic cracking conditions in a second fluid catalytic cracking stage to form a second cracked product, wherein the second fluid catalytic cracking stage is separate from the first second fluid catalytic cracking stage and wherein the catalyst of the second fluid catalytic cracking stage comprises between about 50 and about 95 wt. % shape-selective zeolite, and between about 5 and about 50 wt. % of a large-pore zeolite having a pore diameter greater than or equal to about 0.7 nm.
Another embodiment of the present invention comprises a process for catalytically cracking a cycle oil to selectively increase the yield of light olefins comprising the steps of: (a) contacting a FCC feed with a catalytic cracking catalyst under catalytic cracking conditions in a first FCC reactor to form a first cracked product comprising a cycle oil fraction comprising aromatic species; (b) separating the first cracked product from the catalyst of the first FCC reactor; (c) stripping the catalyst of the first FCC reactor; (d) contacting the catalyst of the first FCC reactor with a gas comprising oxygen; (e) passing the catalyst of the first FCC reactor back to the first FCC reactor; (f) separating at least a portion of the cycle oil fraction from the first cracked product; (g) hydrogenating a substantial portion of the aromatic species in the cycle oil in the presence of a hydrogenation catalyst under hydroprocessing conditions to form a substantially hydrogenated cycle oil, wherein the hydrogenation catalyst comprises at least one Group VIII metal and at least one Group VI metal on at least one refractory support, wherein the Group VI metal is selected from the group consisting of Pt and Pd, wherein the weight of the aromatic species in the hydrogenated cycle oil is less than about 1% of the total weight of the hydrogenated cycle oil; (h) contacting the hydrogenated cycle oil with a catalytic cracking catalyst under catalytic cracking conditions in a separate second FCC reactor to form a second cracked product, wherein the catalyst used in the second FCC reactor comprises wherein the catalyst used in the second FCC reactor comprises between about 80 and about 95 wt. % ZSM-5, and between about 5 and about 20 wt. % of a large-pore zeolite having a pore diameter greater than or equal to about 0.7 nm and a unit cell size less than 24.27 xc3x85; (i) separating the second cracked product from the catalyst of the second FCC reactor; (j) stripping the catalyst of the second FCC reactor; (k) contacting the catalyst of the second FCC reactor with a gas comprising oxygen; and, (l) passing the catalyst of the second FCC reactor to said second FCC reactor back to the second FCC reactor.